JPH0672172A - Driving force distributing device for four-wheel drive vehicle with four-wheel steering system - Google Patents

Abstract

PURPOSE:To improve turning performance and stability at the time of performing four-wheel steering by providing a target yaw rate setting means for setting the target yaw rate to the small value in a yaw increasing tendency and to the large value in the case of suppressing the yaw. CONSTITUTION:The target yaw rate is set by a target yaw rate setting means 9 on the basis of the vehicle speed detected by a vehicle speed sensor 5, a steering angle detected by a steering angle sensor 6 and the steering angle control state controlled by a four-wheel steering system 4 and detected by a four-wheel steering detecting means 8. That is, in the case of detecting the control state of the steering angle in the yaw increasing direction by the four- wheel steering system 4, the target yaw rate is set to the small value. On the contrary, in the case of detecting the control state of the steering angle in the yaw suppressing direction, the target yaw rate is set to the large value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention relates to a four-wheel steering system (4W
The present invention relates to a device for controlling distribution of driving force to front and rear wheels in a four-wheel drive vehicle equipped with S).

[0002]

2. Description of the Related Art As is well known, turning of a four-wheeled vehicle is performed by causing a side slip angle on a running tire and generating a cornering force corresponding to the side slip angle. However, a cornering force generated on a front wheel and a rear wheel is generated. Depending on the size of the, the situation of the turning motion of the vehicle will be different. On the other hand, since the frictional force of the tire, that is, the vector sum of the driving force and the lateral force is substantially constant, the lateral force changes due to the driving force applied to the tire. Therefore, in a four-wheel drive vehicle, the cornering force generated at the front and rear wheels can be appropriately controlled by changing the distribution ratio of the driving force to the front and rear wheels, and therefore, JP-A-61-2296616 and JP-A-62-216616. No. 99213 or Japanese Patent Laid-Open No. 3-1993
In the invention described in Japanese Patent No. 70633, the distribution of the driving force to the front and rear wheels in the four-wheel drive vehicle is controlled so that the actual yaw rate matches the target yaw rate.

[0003]

By the way, since the cornering force of a tire changes not only by the driving force but also by the side slip angle, if the front wheels as well as the rear wheels are steered, the turning characteristics of the vehicle can be changed appropriately. be able to. A four-wheel steering system that performs this type of control changes the steering direction of the rear wheels according to the vehicle speed and the steering angle because the turning characteristics required for the vehicle differ depending on the vehicle state such as the vehicle speed. For example, when the vehicle speed is low, turning is emphasized, reverse-phase steering is performed by steering the rear wheels in the opposite direction to the front wheels, and when the vehicle speed is high, stability is emphasized and the rear wheels are steered in the same direction as the front wheels. In-phase steering is performed.

Conventionally, the distribution control of the driving force to the front and rear wheels by the above-mentioned four-wheel drive device and the control of the rear wheels by the four-wheel steering device are not particularly associated with each other, so that the turning characteristics are not always satisfactory. There were cases where it was not possible. That is, in the case of a four-wheel drive vehicle that controls the distribution of the driving force to the front and rear wheels so that the yaw rate becomes relatively large, when performing the above-described general four-wheel steering control, The yaw rate may be too high,
On the other hand, in a four-wheel drive vehicle that controls the distribution of the driving force to the front and rear wheels so that the yaw rate becomes relatively small, the turning performance during turning acceleration at high vehicle speed may deteriorate.

The present invention has been made in view of the above-mentioned circumstances, and a four-wheel drive vehicle equipped with a four-wheel steering system is associated with four-wheel steering control and drive force distribution control to associate the vehicle with the four-wheel steering system. The purpose is to improve the stability and stability.

[0006]

The present invention is characterized in that it has the structure shown in FIG. 1 in order to achieve the above object. That is, the present invention is a four-wheel steering device-equipped four-wheel drive device including a four-wheel drive device 3 for changing the distribution ratio of the driving force to the front and rear wheels 1, 2, and a four-wheel steering device 4 for changing the steering angles of the front and rear wheels 1, 2. In a driving force distribution device for a driving vehicle, a vehicle speed sensor 5, a steering angle sensor 6, a yaw rate sensor 7, a four-wheel steering detection means 8 for detecting a control state of a steering angle by the four-wheel steering device 4, and the vehicle speed. The four-wheel steering detecting means 8 detects that the target yaw rate is set on the basis of the detection values of the sensor 5 and the steering angle sensor 6, and that the four-wheel steering device 4 controls the steering angle in the direction of increasing the yaw. If the target yaw rate is
Target yaw rate setting means 9 for setting a value smaller than the target yaw rate when the four-wheel steering detection means 8 detects that the steering angle is controlled by the four-wheel steering detection means 8.
And a driving force distribution control means 10 for controlling the distribution of the driving force to the front and rear wheels 1 and 2 by the four-wheel drive device 3 based on the comparison result of the actual yaw rate detected by the yaw rate sensor 7 and the target yaw rate. It is characterized by having.

[0007]

In the four-wheel drive vehicle targeted by the present invention,
The distribution ratio of the driving force to the front and rear wheels 1, 2 is changed by the four-wheel drive device 3 according to the running state of the vehicle. Front wheel 1
The rear wheels 2 are steered by the four-wheel steering device 4 as the vehicle is steered, and the steering of the rear wheels 2 is controlled in the same phase or in the opposite phase depending on the vehicle speed and the steering angle of the front wheels 1. The distribution ratio of the driving force to the front and rear wheels 1 and 2 via the four-wheel drive device 3 is
The driving force distribution control unit 10 controls the actual yaw rate detected by the yaw rate sensor 7 and the target yaw rate set by the target yaw rate setting unit 9 based on the comparison result. The target yaw rate is a target based on the vehicle speed detected by the vehicle speed sensor 5, the steering angle detected by the steering angle sensor 6 and the control state of the steering angle by the four-wheel steering device 4 detected by the four-wheel steering detecting means 8. It is set by the yaw rate setting means 9. That is, when it is detected that the four-wheel steering system 4 controls the steering angle in the direction of increasing the yaw, the target yaw rate is set to a small value, and conversely, the steering angle is controlled in the direction of suppressing the yaw. If it is detected, the target yaw rate is set to a large value. Therefore, the steer characteristic at low vehicle speed does not become the oversteer characteristic or its tendency too much, and the tendency of understeer at high vehicle speed does not become too strong.

[0008]

Embodiments of the present invention will now be described with reference to the drawings. FIG. 2 is a block diagram schematically showing an embodiment of the present invention. The four-wheel drive vehicle shown here is an engine 20.
The output shaft of the transmission 21 connected to the rear wheel 2 via the four-wheel drive device 22 capable of changing the distribution ratio of the driving force.
3 and the front wheel 24. This four-wheel drive 2
2 may be one that has been generally adopted in the related art. For example, differential rotation of front and rear wheels is allowed and torque is distributed through a differential gear or a planetary gear, and the differential action is a multi-disc clutch. It is possible to use a device having a configuration in which the torque distribution ratio is changed steplessly or stepwise by limiting with.

The control for changing the distribution ratio of the torque to the front and rear wheels can be executed directly by controlling the hydraulic pressure for engaging the above-described differential limiting clutch. As a control device therefor, a four-wheel drive electronic control device is used. (4WD-ECU)
25 (hereinafter abbreviated as a 4WD control device) is provided. This mainly includes a central processing unit (CPU), a storage device (RAM, ROM), and an input / output interface (not shown), and performs an operation based on the input data, and as a result, According to the above, the hydraulic pressure of the limited slip differential clutch is changed. As a means for changing the hydraulic pressure of the differential limiting clutch, a widely known hydraulic control device or hydraulic circuit such as a linear solenoid valve or a duty solenoid valve can be adopted. Further, signals for vehicle speed, longitudinal acceleration, lateral acceleration, rotational speeds of front and rear wheels, stop signals, fail signals of various sensors, etc. are input to the 4WD controller 25 as data for control.

The four-wheel drive vehicle shown in FIG. 2 is also equipped with a four-wheel steering device that steers the front wheels 24 or the rear wheels 23 in accordance with the running state of the vehicle, which is picked up from the front wheel steering mechanism 26. A four-wheel steering electronic control unit (4WS) that performs calculation based on the steering angle δ, the actual yaw rate γ detected by the yaw rate sensor 27, and other data such as vehicle speed.
-ECU) (hereinafter abbreviated as four-wheel steering control device) 28
And a rear wheel steering mechanism 29 controlled by the four-wheel steering control device 28. The four-wheel steering control device 28, like the four-wheel drive control device 25 described above, has a central processing unit (CPU) and a storage device (RAM, RO).
M) and an input / output interface (not shown) as main components, and steer the rear wheels 23 in a direction in which yaw is increased, that is, in a phase opposite to that of the front wheels 24, or yaw based on the calculation result. The steering is performed in the restraining direction, that is, in the same phase as the front wheels 24. In addition,
As the rear wheel steering mechanism 29, a structure that drives a steering linkage for the rear wheels to give a steering angle to the rear wheels 23, or a mechanism that moves a suspension bar to give a steering angle to the rear wheels 23 is adopted. be able to.

The four-wheel drive control device 25 and the four-wheel steering control device 28 are connected to each other in a data communicable manner, and the four-wheel steering control device 28 sends the four-wheel drive control device 25 a state of four-wheel steering, that is, The actual yaw rate γ and the steering angle δ are transmitted as well as whether the wheels 23 are in-phase steering or anti-phase steering.

The four-wheel steering control device 28 controls the steering of the rear wheels 23 based on the input data such as the steering angle δ and the actual yaw rate γ, while the four-wheel drive control device 25 controls the actual yaw rate. Based on the input data such as γ, vehicle speed V, and steering angle δ, the difference between the rotational speeds of the front and rear wheels and the deviation between the target yaw rate and the actual yaw rate are obtained, and the clutch hydraulic pressure of the differential limiting clutch is controlled according to these differences. Then, the target yaw rate, which is the basic data for the control, is obtained. Below, an example of the control routine by this 4WD controller 25 will be explained.

FIG. 3 is a flow chart for controlling the clutch hydraulic pressure P _CD of the differential limited clutch which determines the distribution ratio of the driving force to the front and rear wheels. In step 1, the actual steering angle δ obtained from the steering angle and each wheel. Speed v, yaw rate γ, front and rear Gx
, And the lateral Gy are read, then in step 2, the vehicle speed (vehicle speed) V is estimated from the wheel speed v, and the rotation speed N of each wheel is obtained. Further, in step 3, the front wheel rotational speed N _F and the rear wheel rotational speed N _R are calculated as the average rotational speeds of the left and right wheels (N _F = (N _FL + N _RR ) / 2, N _R = (N _RL
+ N _RR ) / 2). From the obtained rotational speeds N _F and N _R of the front and rear wheels, the absolute value ΔN _FR of the rotational speed difference between the front and rear wheels is obtained (step 4). Next, in step 5, the target yaw rate γ ₀ is set.

This target yaw rate is generally the vehicle speed V
It is calculated based on the steering angle δ and the steering angle δ, but in the four-wheel drive vehicle shown in FIG. 2, a value that also takes into consideration the situation of four-wheel steering is adopted. FIG. 4 is a flowchart showing a subroutine for obtaining a target yaw rate according to the situation of four-wheel steering. First, at step 10, the actual steering angle δr of the rear wheels is received from the four-wheel steering control device 28. The actual steering angle δr of the rear wheels is calculated by the following equation: δr = R1 (V) δf + R2 (V) γ Here, R1 (V) is a steering angle proportional coefficient, and the value shown in the map of FIG. 5 is adopted as an example. Δf is the actual steering angle of the front wheels. Further, R2 (V) is a yaw rate feedback coefficient, and as an example, the value shown in the map of FIG. 6 is adopted. And γ is the actual yaw rate. Since the coefficients R1 (V) and R2 (V) have the values shown in FIGS. 5 and 6, the value of the rear wheel steering angle δr at a low vehicle speed becomes negative, and so-called reverse phase steering is performed. Also, at high vehicle speeds, the value of the rear wheel steering angle δr becomes positive and in-phase steering is performed. The situation of such four-wheel steering is judged in step 11, and if the rear wheel steering angle δr is in the direction of suppressing yaw, the routine proceeds to step 12, where the target yaw rate γ of a predetermined value is obtained.
₀₁ is adopted. On the contrary, if the rear wheel steering angle δr is in the direction of increasing the yaw and the judgment result in step 11 is “No”, the process proceeds to step 13 and is smaller than the target yaw rate γ ₀₁ adopted in step 12. The target yaw rate γ ₀₂ (<γ ₀₁ ) of the value is adopted.

These target yaw rates γ ₀₁ and γ ₀₂ are obtained by correcting the target yaw rate γ ₀₃ , which is determined according to the steering angle δf of the front wheels and the vehicle speed V, according to the situation of four-wheel steering.
For example, it is possible to select from a map in which the rear wheel steering angle δr is set as a parameter. It should be noted that γ ₀₃ is γ ₀₃ = K1 (v) · δf, and this coefficient K1 (v) is K1 (v) = V / [L (1 + Kh) V ² ]. Here, L is the wheel base and Kh is the stability factor.

Further, the target yaw rate γ _0n according to the situation of four-wheel steering can be obtained by directly calculating instead of setting it based on the map. If one example of the arithmetic expression is shown, γ ₀ = γ _0n = K3 [sign (γ) · δr] · γ ₀₃ where K3 [sign (γ) · δr] is the vehicle speed V and the front wheel steering angle δf
The sign (γ) is a sign of the yaw rate, which is a coefficient for correcting the value determined by and.

In the flow chart of FIG. 3, the deviation Δγ (= γ (γ ₀ −γ)) between the target yaw rate γ ₀ and the actual yaw rate γ obtained as described above.
Is calculated in step 6. Here, the product of the difference between the target yaw rate γ ₀ and the actual yaw rate γ and the actual yaw rate γ is taken as the deviation Δγ for the following reason. That is, the yaw rate is a directional parameter, and
The deviation Δγ also represents the understeer tendency and the oversteer tendency with positive (+) and negative (−) signs,
If the product is taken as described above, a positive value is obtained when the understeer tendency is present, and a negative value is obtained when the oversteer tendency is obtained, regardless of whether the vehicle is turning to the left or turning to the right.

Deviation Δ of the yaw rate thus obtained
Based on γ, the differential limiting clutch pressure P _CD due to the rotational speed difference ΔN _FR between the front and rear wheels is corrected to control the differential limiting clutch pressure P _CD in consideration of the steer characteristic.

That is, the vehicle equipped with the four-wheel drive device 22 shown in FIG. 2 is a four-wheel drive vehicle based on the rear-wheel drive, and by increasing the differential limitation, the distribution ratio of the driving force to the front-wheel side is increased. Therefore, if there is an understeer tendency, it is necessary to increase the distribution ratio of the driving force to the rear wheels to compensate for the oversteer side. It is necessary to increase the distribution ratio of the driving force to the side to make the understeer tendency. Therefore, based on the map shown in FIG. 7, the correction coefficient K _{2 (Δγ)} corresponding to the deviation Δγ is set to a value smaller than “1” when Δγ is large in the positive direction and large in the negative direction. In this case, set it to a value larger than "1".

On the other hand, the engagement oil pressure P (ΔNFR) of the limited slip differential clutch based on the rotational speed difference ΔN _FR between the front and rear wheels is the rotational speed difference Δ as shown in the map of FIG.
Although it is increased as N _FR increases, if there is a deviation between the actual yaw rate and the target yaw rate during turning, the engagement hydraulic pressure P (ΔNFR ₎ is multiplied by the correction coefficient K _{2 (} Δγ) to correct it. (Step 7), and set the value to the actual engagement hydraulic pressure P _CD.
As a differential limiting clutch. The relationship between the actual engagement hydraulic pressure P _CD and the front-rear wheel rotational speed difference ΔN _FR is expressed by a deviation Δγ.
When is expressed as a parameter, it is as shown in FIG.

That is, the engagement hydraulic pressure P _CD when the yaw rate deviation Δγ is "0" is based on the rotational speed difference ΔN _FR between the front and rear wheels. If there is an oversteer tendency, the correction coefficient K _{2 (Δγ)} having a value larger than “1” is multiplied by the engagement hydraulic pressure P (ΔNFR) due to the rotation speed difference ΔN _FR of the front and rear wheels. The engagement hydraulic pressure P _CD to be applied becomes higher pressure. Therefore, in the four-wheel drive vehicle shown in FIG. 2, the distribution ratio of the driving force to the front wheels becomes high, and the difference in the rotational speeds of the front and rear wheels ΔN.
_In addition to suppressing _FR , oversteering tendency is suppressed, resulting in stable steering characteristics. If there is an understeer tendency in addition to the difference in the rotational speeds of the front and rear wheels, the correction coefficient K _{2 (Δγ)} becomes a value smaller than “1”, which is the engagement hydraulic pressure P based on the difference in the rotational speeds of the front and rear wheels.
Since it is multiplied by (ΔNFR), the engagement hydraulic pressure P _CD to be actually set becomes a lower pressure. Therefore, at the time of turning, even if it is determined that the engagement hydraulic pressure P _CD of the limited slip differential clutch should be increased due to the rotation speed difference ΔN _FR between the front and rear wheels, this is corrected based on the deviation of the yaw rate to a low pressure. Therefore, in the four-wheel drive vehicle shown in FIG. 2, the distribution ratio of the driving force to the rear wheels is increased, and as a result, the understeer tendency is suppressed and the stable steer characteristic is obtained.

Engaging hydraulic pressure P _CD set as described above
Is the actual yaw rate gamma is for distributing the driving force to the front and rear wheels so as to match the target yaw rate gamma _0, the target yaw rate gamma ₀ is the vehicle speed V and the front wheel steering angle δf and four-wheel steering as described above Therefore, the engagement hydraulic pressure P _CD also becomes a pressure corresponding to the situation of four-wheel steering. That is, if the rear wheel steering angle δr is in the direction of increasing yaw, the target yaw rate γ ₀ is set to a small value, and the drive force is distributed to the front and rear wheels so that the actual yaw rate matches the target yaw rate γ _0. Since the target yaw rate γ ₀ is small, it is possible to prevent the steer characteristic from becoming an oversteer characteristic and the understeer characteristic from becoming too weak. Further, if the rear wheel steering angle Δr is in the yaw suppressing direction, the target yaw rate γ ₀ is set to a large value, so that the steer characteristic is prevented from becoming an excessive understeer characteristic. Therefore, the turning performance and stability of the vehicle are improved by performing the control described above.

In the above embodiments, the distribution of the driving force to the front and rear wheels is changed by limiting the differential action of the differential gear mechanism by the clutch. However, the present invention is not limited to this. However, the present invention is not limited to the above-mentioned embodiment, and the point is that the distribution ratio of the driving force to the front and rear wheels may be changed according to the clutch engagement force. Further, the comparison between the target yaw rate and the actual yaw rate may be performed not by multiplying the difference between the two and the actual yaw rate but by taking the ratio of the two. Anything that can be compared so that it can be understood in a simple manner is acceptable.

[0024]

As described above, according to the present invention,
If the yaw tends to increase by steering the four wheels, the target yaw rate becomes a small value, and therefore the distribution of the driving force to the front and rear wheels becomes a distribution with a strong understeer characteristic, and stability is ensured. When the four-wheel steering is performed to suppress yaw, the target yaw rate becomes a large value, and the driving force is distributed to the front and rear wheels so as to weaken the understeer characteristic or the oversteer characteristic. As a result, yaw is not excessively suppressed and the turning performance is improved.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of the present invention in principle.

FIG. 2 is a block diagram schematically showing an embodiment of the present invention.

FIG. 3 is a flowchart showing a control routine of an engagement hydraulic pressure of a limited slip differential clutch.

FIG. 4 is a flowchart showing a subroutine for setting a target yaw rate in accordance with the situation of four-wheel steering.

FIG. 5 is a map of a steering angle proportional coefficient for obtaining a rear wheel steering angle.

FIG. 6 is a map of a yaw rate feedback coefficient for obtaining a rear wheel steering angle.

FIG. 7 is a map showing a relationship between a deviation between a target yaw rate and an actual yaw rate and a hydraulic pressure correction coefficient.

FIG. 8 is a diagram showing a relationship between a rotational speed difference between front and rear wheels and an engagement hydraulic pressure of a differential limiting clutch.

FIG. 9 is a diagram showing the relationship between the rotational speed difference between the front and rear wheels and the actual engagement hydraulic pressure of the differential limiting clutch when there is a deviation between the target yaw rate and the actual yaw rate.

[Explanation of symbols]

 1 front wheel 2 rear wheel 3 four-wheel drive device 4 four-wheel steering device 5 vehicle speed sensor 6 steering angle sensor 7 yaw rate sensor 8 four-wheel steering detection means 9 target yaw rate setting means 10 driving force distribution control means

Claims (1)

[Claims] 1. A drive force distribution device for a four-wheel drive vehicle with a four-wheel steering device, comprising: a four-wheel drive device for changing a distribution ratio of a driving force to front and rear wheels; and a four-wheel steering device for changing a steering angle of front and rear wheels. A vehicle speed sensor, a steering angle sensor, a yaw rate sensor, four-wheel steering detection means for detecting the control state of the steering angle by the four-wheel steering device, and a target yaw rate based on the detection values of the vehicle speed sensor and the steering angle sensor. Set
In addition, the target yaw rate when the four-wheel steering detecting means detects that the four-wheel steering system controls the steering angle in the direction to increase the yaw, and the four-wheel steering system sets the steering angle in the direction to suppress the yaw. Based on the target yaw rate setting means for setting a value smaller than the target yaw rate when the control is detected by the four-wheel steering detection means, and the comparison result between the actual yaw rate detected by the yaw rate sensor and the target yaw rate. A driving force distribution control device for controlling the distribution of driving force to the front and rear wheels by the four-wheel drive device, the driving force distribution device for a four-wheel drive vehicle with a four-wheel steering device.